This invention relates to a closure arrangement for spent nuclear fuel shipping or storage containers.
In the transportation and storage of spent nuclear fuel, it is necessary to use containers for the fuel that prevent the escape of radiation, and that retain their integrity under extreme stresses arising from, for example, their accidentally being dropped, such as may occur in a train derailment. It is imperative that the container, often referred to as a cask, be so constructed that harmful radiation, including radioactive gas, does not escape, and that the heat generated by radioactive decay of the spent fuel is adequately dissipated.
Perhaps the most vulnerable area in a shipping or storage cask is the opening therein through which the spent nuclear fuel is introduced into the cask, or removed therefrom. Ideally, a closure arrangement for the opening should have the same structural integrity as the remainder of the cask under conditions of extreme stress, and should not, under any conditions, permit escape of radioactive gasses from the cask interior.
To achieve these ends numerous types of closure structures for nuclear fuel casks have been designed and used, examples of which are shown in U.S. patents 4,274,007, 4,445,042, and 4,528,454 all of Baatz et al, 4,302,680 and 4,330,711 of Abner et al, 4,488,048 of Bienek et al, and 4,636,645 of Kessinger.
The Baatz et al patents all show dual closure arrangements wherein the inner closure member has a frusto-conical shape and bears against a plurality of sealing rings. Both the inner and outer closure are held in place by a plurality of bolts extending through the closures from the top thereof into the body of the cask. Such an arrangement has several drawbacks, among which are the necessity of orienting the closures so that the bolt holes in the closures and the threaded bolt holes in the body are aligned; the necessity of screwing the bolts in from the top, thereby putting the operator in the direct line of any radioactive leakage; the time consumed in aligning the bolts and screwing them in; and the galling of the bolts due to the high degree of friction involved.
Abner et al patent 4,302,680 shows a closure system having a stationary plug member and a bayonet locking movable member which, when locked, applies force against the plug member to hold it in place. Patent 4,330,711 of Abner et al shows a similar arrangement, with a slightly different configuration of the bayonet locking hold down member. Both of the Abner et al patents rely, therefore, on the bayonet locking principle, which, while affording good locking under normal conditions is subject to slippage or unlocking under high stress such as may arise in an accident.
Bienek et al show an arrangement for mounting an inner container or cask within an outer container whereby the inner container and outer containers are held in fixed relationship by frictional engagement of their walls with intermediate spacing members. Of interest in their arrrangement is the locking arrangement shown in FIGS. 15 and 16 of the patent wherein a plurality of horizontally movable spring loaded lock bolts are pushed by the springs into corresponding openings in the spacing element. Such an arrangement is directed to containing the cask within the outer container and is not a sealing arrangement. In order to achieve locking, the bolts must be pulled back against the springs, necessitating access from above, and the springs themselves do not positively position the bolts. Furthermore, the bolt and hole arrangement requires precise orientation of the lid containing the bolts with the holes in the spacer.
The Kessinger patent discloses a closure arrangement which, like the foregoing prior art arrangements, bolts from the top, thereby requiring the operator to work above the cask, in the potentially most dangerous area.
In all of the foregoing prior art arrangements, access to the cask necessitates placing the operator above the cask in the area of most likely leakage. In addition, where high pressures have built up inside of the cask, loosening the closure could cause it to be blown off, with possible serious injury to the operator, and escaping radioactive gasses could be blown directly into his face. In addition, where a plurality of bolts are used to seal the closure and hold it in place, they must be removed in order to remove the closure, thereby creating the risk that one or more may be lost.
In addition, as pointed out before, with most of the prior art devices a precise orientation of the closure is necessary before the closure can be fastened down.